In the transmission of electrical power there will exist, at the substation level, one or more main power buses, to which are connected a plurality of individual transmission lines which extend outwardly from the substation, forming a portion of a power grid network. Interconnecting the power bus with each individual transmission line is a main circuit breaker and associated relays and switches. The relays may be of various configurations, including mechanical, electro-mechanical, and/or programmable microprocessor-based (computer) relays. These relays for each transmission line control the operation of the main circuit breaker associated therewith. When the relays recognize a fault condition of some kind on the transmission line, the circuit breaker is tripped to interrupt the power on the line, either on a temporary or more long-term basis, depending on the particular fault condition.
During normal operation of any power transmission system, the main circuit breakers and their associated relays and switches will require maintenance or replacement from time to time. In that event, a temporary replacement breaker and its associated relays are switched into the circuit via a power transfer bus. This combination of replacement breaker and associated relays and switches is generally referred to as a bus-tie breaker system. A number of disadvantages, however, are associated with existing bus-tie breaker systems. Typically, a single bus-tie breaker system must be able to replace temporarily any one of a plurality of in-place main breaker systems, each one having different relay settings. Existing bus-tie breaker systems use primarily electro-mechanical devices which in themselves include no operator capability for relay setting adjustment. Multiple relays which are connected to special switching circuits and ratio-changing transformers make it possible for an operator to provide a small range of relay settings so that the bus-tie breaker can temporarily replace any one of the plurality of main circuit breaker combinations.
However, the extra relays, complicated switching circuits and ratio-changing transformers make the existing bus-tie breaker relay systems complicated, expensive, and bulky. A traditional bus-tie breaker system could fill two or three control panels at the substation, each control panel being 90 inches high and 24 inches wide.
Further, such existing systems are clearly recognized in the industry as being a compromise relative to protection. Ideally, the relay settings of a bus-tie breaker system should be adjustable to match closely the relay settings of the particular breaker combination being temporarily replaced. However, this is not possible with existing bus-tie breaker relay systems and hence, there will, as indicated above, typically result severe compromises with respect to adequate matching.
Hence, in summary, due to poor matching capability, system complexity and otherwise limited functional capability of existing bus-tie relay systems, the power transmission system is at a potential operating disadvantage when an existing bus-tie breaker is connected in the system. In addition to operating disadvantages, the high cost and significant space requirements of existing systems are also important considerations.
It is thus highly desirable that a bus-tie breaker system have the capability of providing a good match with the breaker relay system it is intended to temporarily replace, as well as providing all the other typical functions thereof. While programmable, microprocessor-based relays are known which could in a broad sense provide adequate matching capability, the use of such an existing apparatus in a bus-tie breaker system would require an on-site, highly skilled technician capable of reprogramming the relay for each use. This would be a lengthy and expensive process, and would be particularly disadvantageous when the use of the bus-tie breaker was required at unexpected times.